iPhone6 dead after repair and rare behavirour

Hello, my wife's phone suffered an accident resulting with the screen destroyed and the metal case severely damaged, but it still worked. I bought a new metal outer case, screen and battery, I unmounted all the parts from the old case and put them in the new one, it worked well except the loudspeaker. I put a new loudspeaker and it did not work, next I changed the charging connector ribbon. When I tested the phone was dead. I left it to continue working on the phone the next day, without the battery connector plate and without the screen connectors metal plate mounted. Next day first thing I did was connecting the battery charger and I the screen worked. Next I turned the phone on and it worked. I made a call to my wife's call and the ring worked, apparently everything was OK!. I switched it off and reinstalled the two unmounted plates, I closed the phone and since then is dead. I have checked for loose connectors, I removed both plates, I have done the force trick with the power and home button pressed for ten secs but nothing, any idea? thanks.